1. Field of the Invention
The invention relates generally to methods of fabricating material structures on a substrate, including for example homostructures and/or heterostructures. More specifically, the invention relates to the field of providing a substrate for epitaxial growth of films for device fabrication.
2. Description of the Related Art
Products such as semiconductor devices are often fabricated in an epitaxial layer of a crystalline semiconductor material grown on a substrate crystal. FIG. 1A shows a perspective view of a semiconductor wafer 10 with devices 20 fabricated in an epitaxial layer (only representative devices 20 are labeled in FIG. 1A, for clarity of illustration). A cross-sectional view of a portion A in FIG. 1A, is shown in FIG. 1B. Wafer 10 includes a substrate 12 and epitaxial layer 14. A cross-sectional view of a portion B in FIG. 1B, is shown in FIG. 1C. Substrate 12 is seen as composed of crystalline units 11, while epitaxial layer 14 is seen as composed of crystalline units 13 that stack with the crystal matrix of substrate 12 (only representative crystalline units 11 and 13 are labeled in FIG. 1C, for clarity of illustration). The combination of an epitaxial layer on a substrate is sometimes referred to as a “material structure” herein. It should be mentioned here that the crystalline units may be single atoms or repeating molecular units, and that the number of layers and type of stacking shown are for illustration only (for example, a material structure may be a homostructure or a heterostructure as these terms are known in the art). Actual substrates, epitaxial layer materials, thicknesses and growth parameters will vary according to design of a final, fabricated product.
3. Crystallography Background Information
FIG. 2 shows a unit cell 32 of a crystalline structure with a variety of planes thereof illustrated and designated in terms of basic Miller indices, which are a standard form of notation in crystallography to designate particular planes of a crystal. Each of three indices x, y and z indicate a direction that is normal to the designated plane, relative to the Cartesian coordinate system x, y, z as shown. For example, plane 34 is an (001) plane, plane 36 is a (100) plane, plane 38 is an (010) plane, plane 40 is a (101) plane, planes 42 are (111) planes, and planes 44 are (1 −1 1) planes. It is conventional to utilize a bar over a Miller index to denote a negative number, and such notation is utilized in the drawings herein, but minus signs are utilized in the text of this specification to show negative numbers, to avoid errors in published documents. Also, by convention, adjacent numerals denote Miller indices when each index is a single digit, but dots are utilized as separators herein when any such index exceeds a value of 9 (e.g., (11•8•11) as seen below).
Unit cell 32 is shown as having simple cubic symmetry (e.g., a lattice constant that is the same in each of the x, y, and z directions). It is convenient, however, when discussing materials having hexagonal structure to utilize a related, four-index Bravais-Miller notation. In Bravais-Miller notation, the y index is essentially rotated relative to its Cartesian equivalent, to follow one of the crystal directions, and a redundant index i is formed from the x and y indices, with i=−x−y. FIG. 3 illustrates the coordinate system of Bravais-Miller notation. Bravais-Miller notation places i after x and y, but before z; therefore, for example, planes denoted as (1 −1 0) and (1 1 0) become (1 −1 0 0) and (1 1 −2 0), respectively, in Bravais-Miller notation. It is therefore understood that three-index notation herein is simple Miller notation while four-index notation is Bravais-Miller notation.
4. Background Information of Silicon Carbide
Silicon Carbide, SiC, is a well known crystalline material that exhibits polytypism, that is, it exists in a variety of crystalline forms that differ in physical arrangement without varying in stoichiometry. SiC that forms a cubic lattice structure is generally designated as β-SiC while SiC that forms either a hexagonal or a rhombohedral structure is generally designated as α-SiC. Each polytype of SiC includes double layers of silicon and carbon atoms tetragonally bonded, such layers can stack relative to one another in three ways to form planes commonly referred to as A, B and C planes.
Another form of notation often applied to SiC polytypes indicates the specific crystalline type and number of layers in a repeating structure of the crystal. The crystalline types are designated as H (hexagonal), C (cubic) or R (rhombohedral). Three of the most common polytypes are the 3C—SiC, 4H—SiC and 6H—SiC polytypes. A 15R—SiC polytype is also relatively common, but has traditionally been viewed as an unusable byproduct of 4H—SiC or 6H—SiC crystal production. Not only has 15R—SiC historically been viewed as useless, significant efforts have been made to prevent its formation during processing.
5. Background Information of Beta Cells and Icosahedral Boron Arsenide (IBA)
Beta cells are known examples of semiconductor devices that may be fabricated utilizing epitaxial layers. Beta cells are capable of the direct conversion of nuclear into electrical energy. The beta cell receives beta particles emitted by some source of radioactive energy; the beta particles excite electron-hole pairs that are separated by an electric field across the semiconductor junction. This creates current that can be used as a source of electrical power. Unlike common batteries or other chemical-based energy sources, beta cells may last a considerable amount of time (e.g., corresponding to a half-life of the radioactive source, often decades or more) making them ideal for situations where a long-term power source is needed or battery changing is impractical, such as in heart pacemakers, satellites, and other electrical systems.
Silicon-based beta cells have been developed, but such beta cells degrade relatively quickly due to radiation damage. Because of this, alternative materials were investigated. Icosahedral boron arsenide B12As2 (IBA) is a wide band gap semiconductor (3.47 eV) with the extraordinary ability to “self-heal” radiation damage, making it an attractive choice. See, e.g., U.S. Pat. No. 6,749,919 issued to Aselage et al. IBA is a member of the icosahedral borides family, which also includes boron carbide, alpha-boron, and icosahedral boron phosphide. IBA may be epitaxially grown on a substrate for use in beta cells, and the lattice constant of IBA is a close enough match to that of SiC (that is, the IBA lattice constants match appropriate multiples of the SiC lattice constant, as discussed below) to consider SiC as an appropriate substrate for IBA growth.
In addition to possible uses as beta cells, IBA also shows promise for use in other applications where it is desirable to obtain an electrical signal from neutrons emitted from radioactive sources such as a neutron detector. Neutron detectors are useful as an indicator of the presence of radioactive materials, e.g., for security or regulatory compliance purposes. Another potential application of IBA is in thermoelectric converters.